This invention relates to a multilayer filter having characteristics of a band pass filter for use in mobile communication equipment such as a portable cellular telephone and the like.
A typical conventional multilayer filter comprises a plurality of strip-line resonators in the form of a multilayer body which is generally formed from dielectric and conductive layers which are stacked up by a sheeting or screen printing method before being sintered. In order to reduce the size of the multilayer filter using the strip-line resonators, the resonance frequency is lowered by providing capacitors connected in parallel in the multilayer body to obtain target filter characteristics.
In such a multilayer filter as formed with the strip-line resonators, however, current is concentrated on the edge portion of the strip-line conductive layer and the Q-factor is degraded, which poses a problem in that good filter characteristics are unobtainable.
It has been proposed by JP-A 9-35936 to use through-hole electrodes as inductance elements for solving the foregoing problems.
The multilayer filter disclosed in the aforesaid Japanese Patent Publication is seemingly intended to set the ratio W/d of the diameter d of a through-hole to the width W of a multilayer body is set at about 13. With an arrangement like this, however, the Q-factor would never be improved because the resistance value grows larger, though a large inductance value can be secured.